1. Field of the Invention
The present invention relates to a thermal activation apparatus for a heat-sensitive adhesive sheet in which a heat-sensitive adhesive layer that usually exhibits non-adhesiveness and exhibits adhesiveness when thermally activated by heating is formed on one surface of a sheet-like substrate, and a printer provided with the thermal activation apparatus.
2. Related Background Art
Up to now, as disclosed in JP 11-79152 A, a heat-sensitive adhesive sheet having a heat-sensitive adhesive layer that exhibits adhesiveness by being heated has been put into practical use. Such a heat-sensitive adhesive sheet has advantages in that the sheet before being heated can be handled easily because there exists no adhesiveness, industrial waste is not produced since a peeling sheet is not required, and the like. In order to exhibit the adhesiveness of the heat-sensitive adhesive layer of the heat-sensitive adhesive sheet, the heat-sensitive adhesive layer may be heated by using a thermal head generally used as a recording head of a thermal printer. Further, in the case where a heat-sensitive recordable layer is provided on a surface of the heat-sensitive adhesive sheet on opposite side of the heat-sensitive adhesive layer, recording and thermal activation can be performed with a similar thermal head.
A platen roller provided so as to be opposed to a thermal head in an ordinary thermal printer is made of dimethylsilicon rubber having a small permanent deformation. The dimethylsilicon rubber has a rubber hardness of about 30 to 60 degrees. In order for the platen roller to support a recording medium as an underlying member during recording, it is preferable that rubber be crushed to some degree, and for this purpose, the platen roller is pressed to the thermal head under a relatively large pressure of 20 gf/mm2 or more. Further, as the rubber hardness is higher, the pressure with which the platen roller is pressed to the thermal head is set to be larger so as to ensure the crushed amount of rubber. The configurations of a thermal head and a platen roller similar to those of such a conventional thermal printer are often used in a thermal activation apparatus without any modification.
A printer has been developed, in which a desired character, number, image, or the like is recorded on a recordable layer of a heat-sensitive adhesive sheet, a heat-sensitive adhesive layer is allowed to exhibit adhesion under the condition that the heat-sensitive adhesive sheet is cut into a predetermined length, and the heat-sensitive adhesive layer is attached to a product, for example, to produce an adhesive label displaying a price, a product name, or the like (see in JP 2003-316265 A, JP 3329246 B and JP 2004-10710). Such a printer includes a recording apparatus for recording a desired character, number, symbol, or image on a recordable layer, and a thermal activation apparatus for thermally activating a heat-sensitive adhesive layer to exhibit adhesion. Such a printer further includes a transport mechanism for transporting a heat-sensitive adhesive sheet, and a cutter mechanism for cutting the heat-sensitive sheet into a desired length to obtain a label. The recording apparatus and the thermal activation apparatus are provided with thermal heads having substantially the same configuration, and platen rollers for supporting and transporting the heat-sensitive adhesive sheet are placed so as to be opposed to the thermal heads, respectively.
In the above-mentioned thermal activation apparatus, it is necessary that a heat-sensitive adhesive sheet is transported by a rotation of a platen roller while adhesion is exhibited by heating a heat-sensitive adhesive layer of the heat-sensitive adhesive sheet by a thermal head. However, in the case where a portion not heated exists in the heat-sensitive adhesive layer of the heat-sensitive adhesive sheet, the friction resistance of a non-heated portion is large, which may cause a transport defect. More specifically, a heated portion, i.e., activated portion, of the heat-sensitive adhesive layer has fluidity immediately after being heated, so that the heated portion can travel smoothly owing to the slipperiness on the surface of the thermal head. However, the non-heated portion, i.e., non-activated portion, has poor slipperiness, and rubs against the surface of the thermal head to cause a defect. For example, in the case where the activated portion and the non-activated portion are arranged in a longitudinal direction, i.e., transport direction, of the heat-sensitive adhesive sheet, the speed of the non-activated portion becomes lower than that of the activated portion, and the non-activated portion stagnates, which is likely to cause skew. Further, in the case where the activated portion and the non-activated portion are arranged in a width direction, i.e., direction orthogonal to the transport direction, of the heat-sensitive adhesive sheet, when the non-activated portion is pressed to the thermal head with pressure, the speed of only that portion becomes lower and that portion stagnates, which is likely to cause jamming. In particular, this tendency becomes remarkable in a high-temperature and high-humidity environment, in which a solidified heat-sensitive adhesive melts.
As described above, the slipperiness of the non-activated portion of the heat-sensitive adhesive layer is poor, and the platen roller idles to cause the stagnation of the heat-sensitive adhesive sheet. This is caused by the larger friction resistance between the non-activated portion and the thermal head than that between a surface, i.e., recordable layer, on an opposite side of the heat-sensitive layer and the platen roller.
In particular, the configuration of the above-mentioned conventional thermal printer is predicated on the transport of a sheet having no heat-sensitive adhesive layer. In the case of using this configuration in the thermal activation apparatus without any modification, a problem of a transport defect of the above-mentioned heat-sensitive adhesive sheet is likely to occur. In other words, irrespective of the magnitude of a pressure with which the platen roller is pressed to the thermal printer, the friction force acting between the non-activated portion of the heat-sensitive adhesive layer and the thermal head over a substantially entire range is larger than the friction force acting between the recordable layer and the dimethylsilicon rubber, of which platen roller is made. Therefore, it is extremely difficult to smoothly transport the non-activated portion of the heat-sensitive adhesive layer without allowing it to stagnate on the surface of the thermal head, by the rotation of the platen roller.
The object of the present invention is to provide a thermal activation apparatus in which a heat-sensitive adhesive sheet having a heat-sensitive adhesive layer on one surface can be transported smoothly without stagnating on the surface of a thermal head, even if a non-activated portion exists in the heat-sensitive adhesive layer, and a printer including the thermal activation apparatus.